1. Field of the Invention
This invention relates to a deformation prevention structure of gradient magnetic field coils in a nuclear magnetic resonance (NMR) apparatus.
2. Description of the Prior Art
In a conventional NMR apparatus for medical use, a pair of saddle-like gradient field coils disposed in such a manner as to cross orthogonally a static field in a bore formed inside a cryostat and face each other in X - Y directions of the section are directly fixed onto a cylindrical support member or bobbin made of an epoxy resin or the like having extremely great resistivity, as disclosed in Japanese Patent Laid-Open No. 279238/1986. A periodical pulse current is caused to flow through the gradient field coils to generate a periodical gradient field inside the bore in order to obtain a three-dimensional sectional image signal of a subject. When this pulse current is caused to flow, electromagnetic force acts on a lead wire through which the current flows in a direction crossing transversely the static field inside the bore in accordance with the Fleming's lefthand rule and a force that causes deformation of the gradient field coils acts. Accordingly, a pulse-like load is applied to the bobbin. Since this load causes oscillation of the bobbin and generates noise, a vibration-proofing rubber is sandwiched between the bobbin and the gradient field coils and the gradient field coils are further bound onto the vibration-proofing rubber by a band.
In a conventional NMR apparatus for medical use, on the other hand, a pair of saddle-like gradient field coils are directly fixed onto a bobbin as a first cylindrical support member made of a material having extremely great resistivity such as an epoxy resin, as disclosed in Japanese Patent Laid-Open Nos. 261105/1987 and 229906/1987. The gradient field coils are disposed in a uniform static field inside the bore and a periodical pulse current is caused to flow through the gradient field coils in order to conduct two-dimensional sectional image processing of a subject inside the bore and to provide a periodical gradient field distribution inside the static field. Accordingly, when the pulse current is passed, electromagnetic force in a radial direction occurs at a peripheral portion of the gradient field coils due to the static field.
Mention can be made of Japanese Patent Laid-Open Nos. 158047/1988 and 239503/1987 as other prior art techniques for supporting the gradient field coils.
In the NMR apparatus described above, the gradient field coils undergo deformation by a quantity corresponding to contraction due to the load of the vibration-proofing rubber sandwiched between them, the gradient field coils themselves oscillate due to insufficiency of rigidity in directions other than a tensile direction, changes occur in the magnetic field distribution inside the bore which is generated precisely by the gradient field coils and distortion of an image different from an actual image occurs in the final image.
Since the gradient field coils are directly fixed onto the bobbin in the NMR apparatus described above, the bobbin causes oscillation due to the pulse-like load resulting from the electromagnetic force and generates great noise when the pulse current is passed, thereby imparting offensive feel to the subject. The noise generated in this case ranges from a low frequency of 250 Hz to a high frequency fo 8 KHz.